dart2js: Don't return `null` from void functions.

pkg/js_ast/lib/src/template.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_ast;
 
 class TemplateManager {
   Map<String, Template> expressionTemplates = new Map<String, Template>();
   Map<String, Template> statementTemplates = new Map<String, Template>();
 
@@ skipping 747 matching lines @@
       containsInterpolatedNode.contains(node);
 
   void visit(Node node) {
     node.accept(this);
   }
 
   void visitNode(Node node) {
     int before = count;
     node.visitChildren(this);
     if (count != before) containsInterpolatedNode.add(node);
-    return null;
   }
 
   visitInterpolatedNode(InterpolatedNode node) {
     containsInterpolatedNode.add(node);
     if (node.isNamed) holeNames.add(node.nameOrPosition);
     ++count;
   }
 }